This laboratory has been studying the complex of enzymes synthesizing deoxyribonucleotides which is induced after bacteriophage T4 infection. This complex is integrated with DNA synthesis so that certain of these enzymes have a second direct role in DNA replication and ribonucleotides are channeled to the DNA growing point. The system intrinsically synthesizes dTMP and HmdCMP derivatives in the ratio of 2:1, i.e., in direct proportion to the Thy/HmCyt ratio in T4 DNA, whether or not DNA is being formed. DNA agents inhibit deoxyribonucleotide synthesis. The nature of this inhibition is being studied. DNA has an intimate, apparently structural role in the regulation. Efforts to isolate the enzyme complex are in progress.